A piezoelectric motor uses a piezoelectric vibrator to transduce electrical energy into kinetic energy that the motor transmits to a moveable body to which the motor is coupled. The motor is generally coupled to the body by resiliently pressing the motor to the body so that a surface region, hereinafter referred to as a “coupling surface”, of the piezoelectric vibrator contacts a surface region of the body. Electrodes in the motor are electrified to excite vibrations in the vibrator that cause the coupling surface to vibrate. Motion is transmitted from the vibrating coupling surface to move the body by frictional forces between the coupling surface and the surface region, hereinafter a “contact surface”, of the body to which the coupling surface is pressed. A suitable elastic element, generally a coil or leaf spring, provides the “coupling force” that resiliently presses the motor coupling surface to the contact surface of the body.
Typically, a coupling surface of a piezoelectric motor comprises a shaped protuberance, a “friction nub” made from a hard, wear resistant material that is mounted to a surface of the motor. Friction nubs may be formed from Alumina, a high impact plastic such as PEEK (polyethyl ethyl ketone) or a cermet alloy. “Cermet” refers to an alloy formed by bonding ceramic particles with a metal such as, for example, Ti, W, Ta or Mo. Cermet alloys of different composition and characteristics are well known in the art and are used, for example, for bearings, seals and cutting tools.
For a given motor coupling surface, such as a friction nub, used to transmit energy from a piezoelectric motor to a moveable body, the nub and the contact surface of the moveable body to which the nub is pressed undergo relatively rapid mutual wear during an initial run-in period of use. Wear during the run-in period engenders mutual shaping and surface conditioning of the nub and motor contact surface that fine tunes matching of the nub and contact surface to each other. As the fine tune matching of the surfaces progresses, control of motion and positioning of the body by the piezoelectric motor improves and stabilizes, and rate of wear of the surfaces asymptotes to a relatively moderate, steady state wear rate.
U.S. Pat. No. 5,453,653, the disclosure of which is incorporated herein by reference, describes a piezoelectric motor having a friction nub bonded to an edge surface of a thin rectangular piezoelectric vibrator having relatively large parallel planar face surfaces and narrow edge surfaces. Electrodes on the face surfaces of the vibrator are electrified to excite and control vibrations in the vibrator and thereby in the vibrator friction nub in a plane parallel to the planes of the face surfaces. An elastic element that applies force to an edge surface of the vibrator opposite the edge surface to which the friction nub is bonded provides a resilient coupling force that couples the motor friction nub to a contact surface of a moveable body.
U.S. Pat. No. 7,075,211, the disclosure of which is incorporated herein by reference, describes piezoelectric motors comprising a rectangular vibrator having large planar face surfaces and a friction nub on a narrow edge surface of the vibrator that can be excited to generate vibrations in the nub selectively in or perpendicular to the plane of the vibrator.